The present invention relates to internal combustion engines and in particular to an apparatus and method for varying the valve timing pertaining to a combustion chamber thereof.
Improvements in power output, economy, and emissions are obtained by variable valve timing of spark-ignition engines, and numerous different variable valve timing mechanisms have been tried. Variable valve timing also provides benefits for diesel engines, including: improved starting, the use of a lower compression ratio, reduction in diesel "knock," the ability to use lower quality fuels, a raising and flattening of the torque curve, improved fuel consumption, reduced emissions, and better control of scavenging in turbocharged engines.
Improving the fuel consumption efficiency of an internal combustion engine can be accomplished by decreased friction, higher compression ratios, improved combustion, and reduction of an engine's pumping losses. The pumping losses, which are the negative work required by an engine to intake and exhaust gasses during operation, are a significant fraction of the losses which reduce the fuel consumption efficiency of the engine. In the case of a spark-ignition engine, these losses result primarily from the resistance associated with the flow of fresh air past the throttling valve before entering the individual combustion chambers of the engine. The throttle performs the necessary function of controlling the engine power output by varying the amount of air/fuel mixture available for consumption. Thus, any elimination of the throttle valve requires an alternative means of controlling the amount of air/fuel mixture inducted into the combustion chamber to support the required engine load.
A standard spark-ignition automobile engine operates the majority of the time at part throttle where pumping losses are greatest. The penalty in part-load performance of a conventional spark-ignition engine varies from 3.5% of the nominal mean-effective pressure at wide-open throttle to nearly 100% for a fully throttled idling engine. These performance penalties are attributed primarily to the throttling process. It is believed that running an engine at wide-open throttle throughdut its load-speed range would improve the average overall efficiency of the engine by about 20%.
U.S. Pat. No. 4,388,897 to Rosa, proposes a variable valve timing device comprising a camshaft wormed over part of its length and carrying a spittable cam assembly separable along the axis of rotation of the camshaft. However, in the Rosa device the control over the valve event is dependent upon the speed of the camshaft. Moreover, the degree of control over the valve event permitted by the Rosa device is limited by the sensitivity of the linkage that restrains axial movement of the camshaft. Furthermore, the axial shifting principle of the Rosa device renders it difficult to miniaturize to conserve space in the engine compartment.